In the fabrication of semiconductors, silicon wafers are held in a cassette and then moved to various pre-programmed processing locations by a robotic handling system. The latter typically includes a mechanism with degrees of freedom in radial (R), angular (.theta.) and vertical (Z) directions and having a robot arm with a vacuum or edge-gripping wand. The robot must be able to pick up wafers from a storage cassette and then transfer them to a designated station or a plurality of stations where the wafer will undergo some arbitrary process such as heating or alignment. In order to perform these actions, the robot must have precise knowledge of the R, .theta. and Z positions of the wafer at all cassette and station locations. A robot control system must provide the aforesaid knowledge to position the robot arm and thus the gripped wafer precisely within a cassette or process station for each robot function.
To provide proper operation of the robot system when initially set up or when restarted after replacement of a component or when a process location has been changed, the robot must be programmed or "taught" so that for each operation phase, the robot arm is positioned precisely at the proper location for the desired function. Heretofore, this initial and/or subsequent programming or "teaching" step was accomplished by trained personnel using visually estimated trial and error adjustments of the robot mechanism and control.
For example, using conventional controls, a robot was heretofore installed and "taught" by jogging the robot around and, at each process station, recording wafer placement locations with a teach pendant. Besides consuming many hours, this manual procedure introduced subjectivity and thus a significant possibility for errors since no two technicians could set the same positions. This created a problem of reproductivity, that is, of setting the robot in a precise predetermined position for each of a multitude of cycles. Whenever a wafer cassette is not perfectly positioned within specifications or a machine component wears or malfunctions and requires replacement, the robot must be re-taught because it cannot adapt to such variations. If the robot is not re-taught properly within close tolerances, serious damage or loss of expensive wafers could result.
It is therefor a general object of the present invention to provide a method and apparatus for automatically calibrating or teaching a wafer handling robot in conjunction with its control system so that it will operate reliably with reproducibility for a multitude of cycles and within close tolerances to manipulate wafers from cassette holders to various process stations without any damage to wafers.
Further objects of the invention are to provide a wafer-handling automatic calibration system that will automatically calibrate and adjust a wafer handling robot in a relatively short time, for example, after robot components have been removed and replaced.
Another object of the invention is to provide an automatic calibration system for a wafer handling robot which utilizes a machine controller that is programmed to utilize known dimensional data as well as sensor inputs from the robot and the process stations to control robot movements to precise wafer contacting locations.
Still another object of the invention is to provide a robotic wafer handling system having improved reproducibility of the position of the wafer holding wand in the locale where a semiconductor wafer is placed or removed from an enclosure by virtue of the known dimensional data of the wand and enclosure as well as sensor inputs from the robot and the process stations to control robot movement to precise wafer contacting locations.